In recent years, many vehicles are equipped with navigation systems that provide the driver with travel guidance of the vehicle so that the driver can easily arrive at a desired destination. The navigation systems are systems that can detect the current location of the vehicle by a GPS receiver etc. and obtain map data corresponding to the detected current location via a recording medium such as a DVD-ROM or an HDD or via a network to display the map data on a liquid crystal display monitor. The navigation systems also have a route search function to search for, when a desired destination is entered, a recommended route from the location of the vehicle to the destination. When the searched recommended route is set as a guide route, the navigation systems display the guide route on a display screen and provide the user with voice guidance when the vehicle approaches an intersection etc., thereby reliably guiding the user to the desired destination. Recently, some mobile phones, smartphones, tablet computers, personal computers, etc. also have a function similar to that of the navigation systems.
In recent years, in addition to a manual drive mode in which the vehicle travels based on user's driving maneuvers, a drive mode using autonomous driving control has been proposed as a new drive mode, in which the vehicle autonomously travels along a preset route without user's driving maneuvers. For example, in autonomous driving control, the current location of the vehicle, the lane in which the vehicle is traveling, and the position of other vehicles around are detected as needed, and vehicle control such as control of steering, a driving source, and a brake is autonomously performed such that the vehicle travels along a present route. The drive mode using autonomous driving control is advantageous in that it can reduce the burden of driving on the user. In some situations, however, it is difficult for the vehicle to travel by autonomous driving control, depending on the road condition. Examples of such situations include a situation where the vehicle goes around a sharp curve where it is difficult to detect surroundings by a camera or a sensor, a situation where the vehicle travels in a road section with a deteriorated pavement marking, and a situation where the vehicle makes frequent lane changes.
It is therefore desired that the navigation systems etc. search for a recommended route in view of the level of suitability for traveling by autonomous driving control. For example, Japanese Patent Application Publication No. 2004-125726 (JP 2004-125726 A) (pages 6 to 7, FIG. 5) proposes a technique in which, if “use autonomous drive roads” is selected as a preference for a search condition, the navigation system searches for a route preferentially including an autonomous drive road by multiplying route cost of a part corresponding to an autonomous drive road where the vehicle travels by autonomous driving control, such as an exclusive automobile road, by a coefficient smaller than 1.